Enable toolkit_views=1 by default on Mac

As announced on chromium-dev: http://goo.gl/NqrmLk (and after
discussions with chrome-eng-review).

This CL shouldn't change any user-visible UI, just change the way Chrome
is built on Mac. Soon, we will explore running ui/app_list/views/ on Mac
rather than the (outdated) ui/app_list/cocoa/.

A ~0.43% sizes regression on Mac is expected with this CL (~652kB
uncompressed). Full details in a doc: http://goo.gl/oHucmf

With this change, toolkit-views test cases in interactive_ui_tests will
start running on bots. views_unittests will be added to the Mac bots in
a follow-up.

BUG=363529

Committed: https://crrev.com/c11ec57849fd579c7a5559413488c018ea532311
Cr-Commit-Position: refs/heads/master@{#321075}

Review URL: https://codereview.chromium.org/984513002

Cr-Commit-Position: refs/heads/master@{#321294}

build/common.gypi

@@ -197,7 +197,7 @@
           }],
 
           # Set default value of toolkit_views based on OS.
-          ['OS=="win" or chromeos==1 or use_aura==1', {
+          ['OS=="mac" or OS=="win" or chromeos==1 or use_aura==1', {
             'toolkit_views%': 1,
           }, {
             'toolkit_views%': 0,

build/config/ui.gni

@@ -28,7 +28,7 @@ declare_args() {
   use_aura = is_win || is_linux
 
   # True means the UI is built using the "views" framework.
-  toolkit_views = is_win || is_chromeos || use_aura
+  toolkit_views = is_mac || is_win || is_chromeos || use_aura
 
   # Whether we should use glib, a low level C utility library.
   use_glib = is_linux && !use_ozone